Sillenite type compounds (I23 space group) of the general formula (I) A12RO20 where A can be bismuth (Bi); R can be lead (Pb), nickel (Ni), aluminum (Al), titanium (Ti), iron (Fe), silicon (Si), barium (Ba), manganese (Mn), zinc (Zn), cadmium (Cd), calcium (Ca), copper (Cu), gallium (Ga), or vanadium (V), or fractions between 0 and 1 of one or more thereof; and 0 is oxygen, have attracted much interest in solar energy conversion to electricity (photovoltaic application) and conversion of carbon dioxide (CO2) to fuels due to their light absorption capabilities and unique non-centrosymmetry crystal structure (B. Mihailova et al., J. Phys. Chem. Solids, 60, 1829, (1999); W. Yao et al., Appl. Catal. A 243, 185, (2003); J. Zhou et al., Ind. Eng. Chem. Res. 46, 745 (2007); Journal of Molecular Catalysis, 202, 305-311 (2003), with each article herein being expressly incorporated by reference herein in its entirety). Among the sillenite type compounds, bismuth titanate, i.e., Bi12TiO20, also referred to herein as BTO, receives considerable attention for photovoltaic uses because of its high refractive index and electro optic coefficient. Other contemplated uses are photocatalysis application, for example. The Bi12TiO20 crystal is formed by the seven-oxygen coordinated bismuth polyhedra, which is corner shared by other identical bismuth polyhedra and with TiO4 tetrahedra. An enhanced light absorption activity can be attributed to the contribution of 6s electrons of bismuth in the valence band along with O 2p orbitals.
Various methods have been used to prepare bismuth titanate compounds having the resulting Bi12TiO20 sillenite type structure. Such methods include self flux, chemical solution decomposition (CSD), isopropanol-assisted hydrothermal synthesis, co-precipitation methods, and hydrothermal process in potassium hydroxide (KOH) medium using titanium and bismuth sol-gel precursors. In one example, a non-photoactive compound, such as SiO2, can be transformed to a photoactive material having a sillenite type structure using bismuth to form, e.g., Bi12SiO20 (T. Toyoda et al., J Phys D Appl Phys., 19, 909, (1986)). However, the preparation techniques for the aforementioned methods are known to involve multiple steps, high temperatures, and/or complex synthesis procedures. Further, coating the synthesized materials over a suitable conducting substrate can also unfavorably reduce photocatalytic activity.
Also extensively studied and used as photocatalysts to harvest solar energy are nanoparticles of titanium dioxide (TiO2). TiO2 nanoparticles have shown very good stability over a wide pH range and are compatible with other materials, environmentally friendly, inexpensive, and non-toxic. However, interfacial grain boundaries in films prepared from TiO2 nanoparticles have been known to contribute to reducing charge transport by functioning as recombination centers. Recently, the synthesis of TiO2 specifically in the form of hollow nanotubes by anodization of a titanium foil has been demonstrated (D. Gong et al., J. Mater. Res. 16, 3331, (2001)). Such nanotubes are generally produced by anodic oxidation in various electrolytes. Notably, the absence of grain boundaries in the resulting nanotubes favors efficient transport of photogenerated electrons. And since the TiO2 nanotubes are electrically well connected and anchored firmly on an underlying titanium substrate as a raw material for preparing sillenite type compounds, the material is desirable in energy conversion (photovoltaics), environmental remediation (photodegradation), or solar fuel production (CO2 conversion to value added hydrocarbon chemicals such as alcohols, acids, and ethers), for example.
Based on the foregoing, it would be beneficial to provide a simple synthesis process for preparing nanostructures of sillenite type compounds, including Bi12TiO20 nanotubes, from corresponding oxides, e.g., TiO2, which overcomes the aforementioned drawbacks, with the resulting compounds being desirable for use in photovoltaic applications and for solar energy conversion CO2 to fuels, for example.